A raster distortion is one of the important factors in determining the image quality of a color picture tube. Conventionally, pincushion distortions on the upper and lower sides and the right and left sides of the outer periphery have been corrected by a magnetic field of the deflection yoke and a correction circuit of the color picture tube apparatus. Even if these distortions in the upper and lower sides or the right and left sides of the outer periphery are corrected optimally, a pincushion distortion may remain in the longitudinal line or the lateral line in the middle portion of a screen. Such distortions are referred to as “right and left inner pincushion distortion” (shown in FIG. 8) and “upper and lower inner pincushion distortion” (shown in FIG. 9), respectively (“left and right inner pincushion distortion” also may be referred to as “longitudinal line pincushion distortion” or “right and left inner luminescent line distortion,” and “upper and lower inner pincushion distortion” also may be referred to as “lateral line inner pincushion distortion” or “upper and lower inner luminescent line distortion”).
In general, the displacement magnitude X(t) of a luminescent spot on a flat screen is proportional to tan θ where θ is a deflection angle. Therefore, the amount of the horizontal deflection is increased closer to the peripheral portion in the horizontal direction of a phosphor screen. That is, a non-linear distortion called “S-shaped distortion” occurs. This S-shaped distortion is corrected by adding an S-shaped distortion correction circuit. The amount of correction necessary for correcting the S-shaped distortion is inversely proportional to the distance from the deflection center to each spot on the phosphor screen. As shown in FIG. 7, the upper and lower parts of the raster of the color picture tube, i.e., a peripheral portion 72 in the vertical direction of the phosphor screen (hereinafter “peripheral portion” will be referred to) is more distant from the deflection center as compared with the other portions, that is, a middle portion 71 in the vertical direction of the phosphor screen (hereinafter, “middle portion” will be referred to). Accordingly, the amount of the correction necessary for correcting the S-shaped distortion for the peripheral portion may be smaller than that for the middle portion.
In the conventional color picture tube apparatus, although the S-shaped distortion correction circuit can optimize the S-shaped distortion correction in the middle portion in the vertical direction of the phosphor screen, the difference in the necessary correction amount between the middle portion and the peripheral portion in the vertical direction of the phosphor screen has not been considered. Therefore, the S-shaped distortion is over-corrected in the peripheral portion, thus causing the right and left inner pincushion distortion. In particular, a trend toward flattening the front panel of the color picture tube and larger deflection angle have made the right and left inner pincushion distortion significant.
In order to solve such problems, for example, JP 9-149283A discloses a configuration in which a horizontal deflection current is allowed to flow through a saturation reactor and is modulated by a vertical deflection current, thus reducing the total inductance of the horizontal deflection portion when an electron beam is deflected toward the upper and lower portion of the raster, thereby excluding the right and left inner pincushion distortion.
With further flattening of the front panel and larger deflection angle in recent color picture tubes, the difference in the amount of the S-shaped distortion correction between the left or right peripheral portion and the middle portion in the horizontal direction of the raster increases, as well as the difference in the vertical direction of the raster. Consequently, in addition to the right and left inner pincushion distortion, the upper and lower inner pincushion distortion has been a significant problem.
The above-mentioned conventional inner pincushion distortion correction circuit disclosed in JP 9-149283A is an effective means for correcting the right and left inner pincushion distortion, but the means could not correct the upper and lower inner pincushion distortion. Therefore, the conventional means for correcting the distortion only can balance the distortion amount between the upper portion and the lower portion by an upper and lower tilting operation, that is, by tilting the central axis of the deflection yoke in the vertical direction.